linux-kernel - Re: [lkp-robot] [rcutorture] 46e26223e3: WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_stats

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Message-Id: <20180618165646.GA3593@linux.vnet.ibm.com>
Date:   Mon, 18 Jun 2018 09:56:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Joel Fernandes <joelaf@...gle.com>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        mingo@...hat.com, Steven Rostedt <rostedt@...dmis.org>,
        tglx@...utronix.de, lkp@...org
Subject: Re: [lkp-robot] [rcutorture]  46e26223e3:
 WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_stats_print

On Wed, Jun 13, 2018 at 12:29:48AM -0700, Joel Fernandes wrote:
> (I'm actually not working this week, but still thought of replying :))
> 
> On Wed, Jun 13, 2018 at 02:57:11PM +0800, kernel test robot wrote:
> > 
> > FYI, we noticed the following commit (built with gcc-4.9):
> > 
> > commit: 46e26223e39c64763e321f229e324be15179c505 ("rcutorture: Make boost test more robust")
> > url: https://github.com/0day-ci/linux/commits/Joel-Fernandes/rcutorture-Disable-RT-throttling-for-boost-tests/20180611-074731
> > base: https://git.kernel.org/cgit/linux/kernel/git/paulmck/linux-rcu.git rcu/next
> > 
> > in testcase: trinity
> > with following parameters:
> > 
> > 	runtime: 300s
> > 
> > test-description: Trinity is a linux system call fuzz tester.
> > test-url: http://codemonkey.org.uk/projects/trinity/
> 
> I'll try to reproduce this, but it could be because after this patch, the
> boost test is actually working.

Quite possibly.  ;-)

> The reason for the rcutorture test failure could be that the default
> kthread_prio for the system's RCU threads is set to 1 (unless overridden by
> rcutree.kthread_prio) which is also equal to the priority of the rcutorture's
> boost threads. Due to this the rcutorture test could starve the RCU threads
> as well and defeat the boosting mechanism. I was able to solve a similar
> issue by just passing rcutree.kthread_prio of 50 on the kernel command line.
> 
> Paul, would it be ok if we changed the default kthread_prio to something > 1
> so that rcutorture can test properly without needing to pass any extra
> rcutree.* parameters?
> 
> so something like this in kernel/rcu/tree.c ?
> 
> static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 2 : 0;

Would it be possible to also condition this on rcutorture being built
in?  Or are they doing modprobes for rcutorture?

							Thanx, Paul